Continuously variable transmissions (CVTs) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, scooters, all-terrain vehicles (ATV), etc. They often comprise a driving pulley mechanically connected to a motor, a driven pulley mechanically connected to wheels or a caterpillar, possibly through another mechanical device such as a gearbox, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. A CVT changes the ratio within certain limits as required by the operating conditions to yield a desired motor rotational speed for a given driven pulley rotational speed, the latter being generally proportional to the vehicle speed. A CVT may be used with all kinds of motors, for instance internal combustion engines, electric motors, windmills, etc. CVTs can also be used with other machines that are not vehicles.
Each pulley of a CVT comprises two members having opposite conical surfaces, which members are called sheaves. One sheave, sometimes called “fixed sheave”, can be rigidly connected to one end of a supporting shaft while the other sheave, sometimes called “movable sheave”, can be free to slide and/or rotate with reference to the fixed sheave by means of bushings or the like. The conical surfaces of the sheaves apply an axial force on the drivebelt. Moving the sheaves axially relative to each other changes the drivebelt operating diameter, thus the ratio of the CVT.
In order to transmit the motor torque, an axial force has to be applied in the driving and the driven pulleys. These axial forces can be generated by a plurality of possible mechanisms or arrangements. In a legacy mechanical CVT, the axial force in the driving pulley is often generated using centrifugal weights, spring and ramps. In a legacy driven pulley, this force is often generated using cam surfaces and a spring.
Generally, at a low vehicle speed, the operating diameter of the drivebelt at the driving pulley is minimal and the operating diameter at the driven pulley is maximal. This is referred to as the minimum ratio or the minimum ratio condition since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
As the vehicle speed increases, so does the driven pulley rotational speed. For a given operating condition, a certain motor rotational speed is desired, thus a desired ratio can be calculated. The CVT actuation mechanism is provided to set the CVT to the appropriate ratio.
Generally, when the rotational speed of the driving pulley increases, its movable sheave moves closer to the fixed sheave thereof under the effect of an actuation mechanism, for instance a centrifugal mechanism or another kind of actuation mechanism. This constrains the drivebelt to wind on a larger diameter at the driving pulley. The drivebelt then exerts a radial force on the sheaves of the driven pulley in addition to the tangential driving force by which the torque received from the motor is transmitted. This radial force urges the movable sheave of the driven pulley away from the fixed sheave thereof, thereby constraining the drivebelt to wind on a smaller diameter at the driven pulley. A return force, for instance a return force generated by a spring of the driven pulley and/or by another biasing mechanism, often counterbalances the radial force. It may also be counterbalanced by a force generated by the axial reaction of the torque applied by the drivebelt on the driven pulley, which force often results from the presence of a cam system and/or another biasing mechanism that tend(s) to move the movable sheave towards the fixed sheave as the torque increases. A cam system may comprise a plurality of ramp surfaces on which respective followers can be engaged. The followers can be sliding buttons or rollers, for instance. The set of ramp surfaces or the set of followers is attached to the movable sheave. The other set is directly or indirectly attached to the fixed sheave and is in a torque-transmitting engagement with the main shaft supporting the driven pulley. The closing effect of the cam system on the drivebelt tension is then somewhat proportional to the torque received from the motor.
Generally, at the maximum vehicle speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
There are two ways to slow down a vehicle equipped with a CVT. Firstly by reducing the throttle to reduce the engine's torque to decrease the RPM. That affects the centrifugal force on the drive pulley thus reducing pressure on the drive belt. The opening of the drive pulley is also supported by the tension in the drivebelt induced by the pressure applied by the pre-tensed spring of the driven pulley. Secondly, the vehicle is going to slow down if the road load is higher than the available engine's torque, the engine RPM is going to slow down and the drive pulley is going to open. The increased torque applied on the drive belt is going to increase the pinching of the drivebelt by the driven pulley.
Some CVTs are provided with an integrated clutch function. The clutch function can be on the drivebelt or be provided by a mechanism incorporated in the CVT. For instance, when the CVT has a clutch function on the drivebelt, the opposite walls of the fixed sheave and the movable sheave of the rotating driving pulley can be designed to be sufficiently apart that they are not in a driving engagement with the sides of the drivebelt. The drivebelt is then not moving and some models of driving pulleys have a bearing provided between the two sheaves. The outer race of such bearing supports the drivebelt when the driving pulley is in a disengaged position. Then, when the operating conditions are such that clutching is required, the actuation mechanism of the driving pulley moves the sheave walls closer relative to each other. The sheave walls eventually make contact with the sides of the drivebelt. At this point, an axial force is applied by the actuation mechanism on the drivebelt. The amount of torque transferred to the drivebelt is somewhat related to this axial force applied by the actuation mechanism. At one point, enough friction/force is generated between the sheave walls and the drivebelt to produce a significant force transfer between the driveshaft and the drivebelt, thereby causing torque from the motor to be transferred as a driving force on the drivebelt. This driving force is transferred to the driven pulley of the CVT.
Generally, torque applied on the drivebelt will result in vehicle acceleration at some point. The drivebelt will then accelerate in relation to vehicle speed. At start-up, the slippage between the driving pulley sheaves and the drivebelt is high, but decreases as the drivebelt accelerates, to the point where it becomes negligible and the driving pulley is considered fully engaged.
Assisted CVTs are advantageous because they do not relate on the centrifugal force generated by the rotation of the sheaves like legacy mechanically actuated CVTs. In contrast, an electrically actuated CVT, also called assisted CVT or eCVT, uses an electric motor and an adapted gearbox to set the distance between both drive sheaves to set the transmission ratio. This gives the flexibility of using a specific CVT ratio in reaction of predetermined conditions regardless of the centrifugal force applied on the drive pulley. Despite the advantages provided by an assisted CVT, it is appreciated that the assisted CVT can be used in such a manner that it can overheat the drivebelt.
An assisted CVT can be used in many helpful ways, for instance, the operation of an engine at constant speed may be desirable for performing some tasks. The load applied to the engine can vary and therefore have an effect on the speed of the engine. Some types of engines are provided with an engine controller configured to increase the amount of fuel injected in the engine to keep the engine running at substantially constant speed when the load applied thereto increases. However, the engine controller can hardly help sustaining a constant engine speed when a load is applied thereto and even more when the load exceeds the maximum torque the engine can provide at a specific rotational speed. An engine controller maintains a constant engine rotational speed and compensates load variations by managing the throttle until the engine reaches it maximum torque output. Therefore, it might be desirable to use the assisted CVT, which is disposed between the engine and the drive mechanism providing the load applied to the engine, to help the engine remains at constant speed despite significant load variations applied thereon.
Keeping a constant engine rotational speed and use the transmission to change the speed of a machine can be made with a hydrostatic transmission. A hydrostatic transmission consists of a variable-displacement pump and a fixed or variable displacement motor, operating together in a closed circuit. In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank. As well as being variable, the output of the transmission pump can be reversed, so that both the direction and speed of motor rotation are controlled from within the pump. This eliminates the need for directional and flow (speed) control valves in the circuit. Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation. The charge pump is normally installed on the back of the transmission pump and has an output of at least 20% of the transmission pump's output. In practice, the charge pump not only keeps the loop full of fluid, it pressurizes the loop to between 110 and 360 PSI, depending on the transmission manufacturer. A simple charge pressure circuit comprises the charge pump, a relief valve and two check valves, through which the charge pump can replenish the transmission loop. Once the loop is charged to the pressure setting of the relief valve, the flow from the charge pump passes over the relief valve, through the case of the pump or motor or both, and back to tank. Hydrostatic transmissions have nonetheless several drawbacks. Namely their expensive price and their lack of efficiency because of the huge loss in fluid friction.
Thus, a need has been felt for an improved assisted CVT over the prior art. It is therefore desirable to use an assisted CVT in order to emulate a hydrostatic transmission or to keep running an engine at constant speed while managing the output speed of the machine with the assisted CVT. It is also desirable to provide an assisted CVT having a load-limiting mechanism adapted to transfer a limited amount of load to the engine in order to keep the engine running at constant speed. Another need has been felt over the existing art for an assisted CVT adapted to limit the torque transmitted to an engine and adapted to provide a signal and take actions when the assisted CVT reaches its transmission ratio limit. It is also desirable to provide means for preventing drivebelt overheating.